bdiff: normalize the diff (issue1295) When the common part of a diff can be moved forward, move it forward. Otherwise we don't get deterministic results (it would depends on the way we split for the recursion). That way we get identical hunks when doing the same change, it helps to solve issue1295 (inconsistent diffs on different side during a merge).

1 files updated, 0 files merged, 0 files removed, 0 files unresolved
removing b
created new head
% should abort
abort: crosses branches (use 'hg merge' or 'hg update -C' to discard changes)
% should succeed
abort: crosses branches (use 'hg merge' or 'hg update -C')